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<h1 id="JMCOH-GUID-3EC7E6C3-6A64-4659-B99E-CA675A9F6E0A" class="sect1"><span class="enumeration_chapter">4 </span>Frequently Asked Questions</h1>
<div><p>This section lists Frequently Asked Questions about JDK Mission Control.</p>
<div class="section"><p class="subhead1">I cannot connect to the JVM. What could be the problem?</p><div class="p">Consider the following:<ul style="list-style-type: disc;"><li><p>Are you using the correct protocol?</p>
<p>Ensure that you are using the same version of the JVM that you want to monitor as is being used by the JVM running the JMC client.</p>
<p>The format of the service URL is:</p>
<p><tt>service:jmx:rmi:///jndi/rmi://&lt;hostname&gt;/jmxrmi</tt></p>
</li>
<li><p>Are the correct ports opened?</p>
<p>Note that running JMX over RMI requires two ports and that one of the ports will not be known beforehand.</p>
</li>
<li><p>Is the communication caught in a firewall?</p>
<p>For more information, see <a href="GUID-97B37847-8E31-43F2-AA78-E5EBD28EC99A.htm" title="This section details how the JVM is monitored using JMC">JDK Mission Control Communications</a>.</p>
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<div class="section"><p class="subhead1">When attempting to connect to a JVM, I get a stack trace indicating that the JVM attempts to communicate with a strange IP or host name. What does this mean?</p><p>Sometimes RMI can have a problem determining which address to use. This can happen because of any of the following:</p>
<ul style="list-style-type: disc;"><li><p>There are access restrictions in the Security manager.</p>
</li>
<li><p>The machine is multihomed and RMI is selecting the wrong interface.</p>
</li>
<li><p>There is a misconfigured hosts file or a number of different network related configuration problems.</p>
</li>
</ul>
<p>If all else fails, you can try specifying the&#160;<tt>java.rmi.server.hostname</tt>&#160;system property. Note that this can affect applications running on the JVM.</p>
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<div class="section"><p class="subhead1">I am getting exceptions during startup about classes not being found. What is wrong?</p>
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<div class="section"><p class="subhead1">Why does JDK Mission Control fail to find any local JVMs?</p><p>Consider the following:</p>
<ul style="list-style-type: disc;"><li><p>If you are running JMC from Eclipse, make sure that Eclipse is running on a JDK (not JRE).</p>
</li>
<li><p>Make sure that there is a directory named <code>hsperfdata_<span class="codeinlineitalic">username</span></code> in the system's <code>tmp</code> directory, that it is writable by the user running JMC, and that the file system supports access control lists (ACLs).</p>
</li>
</ul>
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<div class="section"><p class="subhead1">How do I connect to JBoss?</p><p>Before starting JMC, you need to add the&#160;<tt>-Xbootclasspath/a</tt>&#160;option to the&#160;<tt>-vmargs</tt>&#160;section of the&#160;<tt>jmc.ini</tt>&#160;file. The value for this option (after a colon) should be set to the extended classpath that is generated by JBoss's&#160;<tt>jconsole.bat</tt>&#160;file (for this, add&#160;<tt>@echo</tt> on&#160;to the file and run it). </p>
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